Initial clinical observations of intra- and interfractional motion variation in MR-guided lung SBRT

Abstract

Objective: To evaluate variations in intra- and interfractional tumour motion, and the effect on internal target volume (ITV) contour accuracy, using deformable image registration of real-time two-dimensional-sagittal cine-mode MRI acquired during lung stereotactic body radiation therapy (SBRT) treatments.

Methods: Five lung tumour patients underwent free-breathing SBRT treatments on the ViewRay system, with dose prescribed to a planning target volume (defined as a 3-6 mm expansion of the 4DCT-ITV). Sagittal slice cine-MR images (3.5 × 3.5 mm² pixels) were acquired through the centre of the tumour at 4 frames per second throughout the treatments (3-4 fractions of 21-32 min). Tumour gross tumour volumes (GTVs) were contoured on the first frame of the MR cine and tracked for the first 20 min of each treatment using offline optical-flow based deformable registration implemented on a GPU cluster. A ground truth ITV (MR-ITV_{20 min}) was formed by taking the union of tracked GTV contours. Pseudo-ITVs were generated from unions of the GTV contours tracked over 10 s segments of image data (MR-ITV_{10 s}).

Results: Differences were observed in the magnitude of median tumour displacement between days of treatments. MR-ITV_{10 s} areas were as small as 46% of the MR-ITV_{20 min}.

Conclusion: An ITV offers a "snapshot" of breathing motion for the brief period of time the tumour is imaged on a specific day. Real-time MRI over prolonged periods of time and over multiple treatment fractions shows that ITV size varies. Further work is required to investigate the dosimetric effect of these results. Advances in knowledge: Five lung tumour patients underwent free-breathing MRI-guided SBRT treatments, and their tumours tracked using deformable registration of cine-mode MRI. The results indicate that variability of both intra- and interfractional breathing amplitude should be taken into account during planning of lung radiotherapy.

Figure 1.

Example sagittal 2D cine-mode imaging during four fractions of treatment for Patient 1. GTV tracking from the off-line deformable image registration is shown for each fraction (inner contour), with the cumulative ITV for the entire fraction also plotted (outer contour ). 2D, two-dimensional; GTV, gross tumour volume; ITV, internal target volume.

Figure 2.

GTV tracking data for Patient 1. (a) Shows AP vs CC motion of the GTV centroid for Patient 1, Fractions 1–4 (first 500 s of each fraction shown), showing interfractional variation in the amplitude of breathing and amount of hysteresis. (b) Shows breathing amplitude as calculated from the vector length displacement of GTV centroid relative to fifth percentile reference position. One minute of data is shown here with the detected inhalation peaks indicated with red circles. (

Figure 3.

Variation in amplitude of breathing, showing amplitude of individual inhalations (grey dots) and boxplots per fraction showing the median (black line), interquartile range (coloured boxes) and min–max range (whiskers) of the amplitudes. The interval endpoints of the medians are displayed as notches in the boxes, representing the 95% confidence interval on the median.

Figure 4.

(a) Maximum intensity projection image of the entire treatment–from a single fraction– showing the MR-ITV_20 min contour, and of the corresponding “best-case” and “worst case” MR-ITV_10 s. (d) Cumulative distributions for the all possible snapshot MR-ITV10 s generated per fraction.